682
FXUS63 KLOT 170535
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1135 PM CST MON DEC 16 2024
KEY MESSAGES
- POTENTIAL FOR LIGHT SNOW ACCUMULATIONS AND TRAVEL IMPACTS
TUESDAY NIGHT INTO EARLY WEDNESDAY MORNING, PRIMARILY NORTH
OF I-80.
- ANOTHER ROUND OF SNOW AND TRAVEL IMPACTS POSSIBLE THURSDAY
INTO EARLY FRIDAY.
DISCUSSION
ISSUED AT 230 PM CST MON DEC 16 2024
THROUGH WEDNESDAY NIGHT:
A COLD FRONT IS SLOWLY PASSING THROUGH THE FORECAST AREA THIS
AFTERNOON. WINDS ARE OUT OF THE SOUTHWEST WITH GUSTS 20 TO 25
MPH, THOUGH THEY ARE EXPECTED TO BECOME MORE WESTERLY IF NOT
NORTHWESTERLY INTO THE EVENING AS THE FRONT EXITS TO THE EAST.
THERE IS A WEAK BOUNDARY TRAVELING EAST OVER SOUTHWESTERN
WISCONSIN. MOST HIGH RES MODELS KEEP IT NORTH OF THE STATE
LINE. IT WOULD NOT BE SURPRISING IF SOME LIGHT DRIZZLE/RAIN
OCCURS ON AREAS JUST SOUTH OF THE STATELINE, BUT CONFIDENCE WAS
LOW ENOUGH TO NOT ADD IT TO THE OFFICIAL FORECAST AND KEEP POPS
AT LESS THAN 15 PERCENT.
QUIETER WEATHER CONDITIONS ARE EXPECTED THROUGH THE OVERNIGHT
AND INTO TUESDAY MORNING. A CLEAR SLOT IN THE CLOUD COVER IS
EXPECTED TO MOVE NORTHEASTWARD AND EXPAND OVER THE FORECAST AREA
REDUCING CLOUD COVER. HOWEVER, IT WILL BE SHORT LIVED AS CLOUDS
FILL BACK IN FROM THE NORTHWEST TUESDAY AFTERNOON AHEAD OF THE
NEXT SYSTEM.
A POSITIVELY TILTED TROUGH WILL DESCEND OUT OF CANADA TUESDAY
NIGHT THROUGH WEDNESDAY MORNING WITH A STRONG UPPER-LEVEL JET
SUPPORTING LARGE SCALE FORCING FOR ASCENT. COLD AIR ADVECTION
WITH THIS WAVE SHOULD ALLOW FOR A COLD ENOUGH THERMAL PROFILE
TO SUPPORT MAINLY SNOW, ESPECIALLY NORTH OF I-80. AS THE CAA
TIGHTENS THE LOW AND MID-LEVEL THERMAL GRADIENT, THE ASSOCIATED
F-GEN MAY POTENTIALLY LOCALLY ENHANCE PRECIP RATES FOR A TIME
LATE TUESDAY NIGHT.
GIVEN THE RUN-TO-RUN CONSISTENCY, POPS WERE MAINTAINED TO
MENTION THE "LIKELY" EXPECTATION OF PRECIPITATION. THE POTENTIAL
FOR FAIRLY NARROW BANDING COMPONENT CERTAINLY POINTS TOWARD
NORTH-SOUTH WIGGLE ROOM IN PLACEMENT OF AXIS OF LIGHT SNOW
ACCUMULATIONS. THAT SAID, THE LIKELY POP PLACEMENT ALIGNS WITH
THE HIGHEST ENSEMBLE POPS. LIMITING FACTORS WILL BE:
RELATIVELY PALTRY COLUMN MOISTURE, THE OVERALL PROGRESSIVE
PATTERN, AND NEAR TO SLIGHTLY ABOVE FREEZING SURFACE TEMPS (SLR
GENERALLY <10:1). ENSEMBLE PROBABILITIES OF >1" SNOW ACCUMS ARE
LESS THAN 10%, SO A COATING TO A HALF INCH IS THE CURRENT
APPARENT UPPER LIMIT, UNLESS MESOSCALE BANDING AIDS IN RELATIVE
OVER- PERFORMANCE. THE SNOW BAND(S) SHOULD SLIDE EAST AND EXIT
FAIRLY EARLY WEDNESDAY MORNING. MAIN TAKEAWAY HERE IS THAT WE'RE
MONITORING FOR SOME WINTRY TRAVEL IMPACTS INTO THE WEDNESDAY
MORNING COMMUTE NEAR AND NORTH OF I-80 AND PRIMARILY INLAND OF
THE LAKE IN NORTHERN ILLINOIS. DRIER CONDITIONS ARE PROJECTED
WEDNESDAY AFTERNOON AND EVENING AS TEMPERATURES BECOME COLDER
BEHIND THE SYSTEM.
DK
THURSDAY THROUGH MONDAY:
NEAR NORMAL HIGH TEMPERATURES ON WEDNESDAY WILL STEP DOWN TO
BELOW NORMAL ON THURSDAY FOLLOWING A RELATIVELY CHILLY WEDNESDAY
NIGHT. ATTENTION IN THIS PERIOD TURNS TO A VIGOROUS ALBERTA
CLIPPER TYPE WAVE THAT WILL LIKELY IMPACT PORTIONS OF THE REGION
THURSDAY AFTERNOON INTO EARLY FRIDAY. AS IS COMMON FOR THESE
LOW-AMPLITUDE FEATURES, PREDICTABILITY IS ON THE LOWER END OF
THE SPECTRUM GIVEN SUBSTANTIAL RUN TO RUN VARIABILITY AND INTRA-
ENSEMBLE MEMBER VARIANCE OF THE ALL IMPORTANT TRACK OF THE
CLIPPER'S SURFACE LOW PRESSURE REFLECTION. LITTLE TO NO SNOW
(TRACK TOO FAR NORTH) TO LIGHT-MODERATE SNOW ACCUMS AND TRAVEL
IMPACTS (INCLUDING TO THE FRIDAY AM COMMUTE) ARE BOTH PLAUSIBLE
OUTCOMES. CHANCE POPS IN THE 30-50% RANGE, HIGHEST NORTHEAST
2/3 OR SO OF THE CWA, APPEAR REASONABLE FOR NOW.
FOLLOWING THE PASSAGE OF THIS WAVE, A STRETCH OF BELOW NORMAL
TEMPERATURES IS IN STORE FRIDAY THROUGH THE WEEKEND AS A DEEP
TROUGH TEMPORARILY ESTABLISHES ITSELF OVER EASTERN NORTH
AMERICA. CHANCES FOR LAKE EFFECT SNOW MAY TAKE AIM AT PORTIONS
OF OUR NORTHWEST INDIANA COUNTIES, HOWEVER CONFIDENCE IN THIS
REMAINS LOW AT BEST CURRENTLY.
BAKER/CASTRO
AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 1135 PM CST MON DEC 16 2024
FORECAST CONCERNS INCLUDE:
- MVFR CIGS OVERNIGHT THROUGH AT LEAST LATE TUESDAY MORNING.
- POSSIBLE IFR CIGS/VSBY WITH LIGHT SNOW EARLY WEDNESDAY MORNING.
AN EXPANSIVE AREA OF MVFR CIGS EXTENDS ACROSS MUCH OF MN/WI AND
IS STEADILY MOVING SOUTHEAST CURRENTLY. THE LATEST GUIDANCE
BRINGS THESE MVFR CIGS ACROSS ALL OF THE TERMINALS OVERNIGHT AND
GIVEN CURRENT TRENDS, THIS LOOKS REASONABLE. HAVE INCLUDED MVFR
CIGS THROUGH AT LEAST MID MORNING WHEN THEY MAY LIFT TO
LOW VFR OR TEMPORARILY SCATTER. IF CIGS REMAIN BKN MVFR INTO
THE AFTERNOON, THEY MAY BRIEFLY SCATTER OUT BY TUESDAY EVENING.
MUCH OF THE SHORT TERM GUIDANCE IS SHOWING A BAND OF LIGHT SNOW
DEVELOPING NORTHWEST OF THE AREA TUESDAY EVENING AND THEN
SPREADING ACROSS NORTHERN IL EARLY WEDNESDAY MORNING. ITS
POSSIBLE THIS BAND IS FAIRLY NARROW, WHICH MAY LIMIT HOW FAR
SOUTH IT MOVES AND ALSO BE SHORTER IN DURATION, PERHAPS 3-4
HOURS IN ANY ONE LOCATION. WHILE THERE REMAINS UNCERTAINTY,
CURRENT TRENDS SUPPORT TEMPO MENTION AT THE END OF THE RFD TAF,
AND PROB MENTION FOR THE 30 HOUR ORD/MDW TAFS. IFR CIGS/VSBY
WILL BE POSSIBLE WHEREVER THIS BAND SETS UP.
SOUTHWEST WINDS 10-15KTS WILL TURN MORE WESTERLY OVERNIGHT AND
PERIODIC GUSTS WILL CONTINUE TO BE POSSIBLE THROUGH MIDDAY
TUESDAY. SPEEDS WILL DIMINISH WITH SUNSET TUESDAY EVENING AS
HIGH PRESSURE MOVES ACROSS THE AREA. WINDS ARE EXPECTED TO
BECOME LIGHT NORTH/NORTHWEST EARLY WEDNESDAY MORNING.
CMS/CASTRO
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 3 AM CST TUESDAY FOR WINTHROP
HARBOR IL TO GARY IN.
SMALL CRAFT ADVISORY UNTIL 9 AM CST TUESDAY FOR GARY TO BURNS
HARBOR IN-BURNS HARBOR TO MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
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